Methane-water

Commercial-scale processes have been developed for the production of hydrogen sulfide from heavy fuel oils and sulfur as well as from methane, water vapor, and sulfur. The latter process can be carried out in two steps reaction of methane with sulfur to form carbon disulfide and hydrogen sulfide followed by hydrolysis of carbon disulfide (116). 

Here we present and discuss an example calculation to make some of the concepts discussed above more definite. We treat a model for methane (CH4) solute at infinite dilution in liquid under conventional conditions. This model would be of interest to conceptual issues of hydrophobic effects, and general hydration effects in molecular biosciences [1,9], but the specific calculation here serves only as an illustration of these methods. An important element of this method is that nothing depends restric-tively on the representation of the mechanical potential energy function. In contrast, the problem of methane dissolved in liquid water would typically be treated from the perspective of the van der Waals model of liquids, adopting a reference system characterized by the pairwise-additive repulsive forces between the methane and water molecules, and then correcting for methane-water molecule attractive interactions. In the present circumstance this should be satisfactory in fact. Nevertheless, the question frequently arises whether the attractive interactions substantially affect the statistical problems [60-62], and the present methods avoid such a limitation. 

Fig. 9.4. Pa (e) and (e) as a function of the binding energy. The simulations treated 216 water molecules, utilizing the SPC/E water model, and the Lennard-Jones parameters for methane were from [63]. The number density for both the systems is fixed at 0.03333 A 3, and T = 298 K established by velocity rescaling. These calculations used the NAMD program (www.ks.uiuc.edu/namd). After equilibration, the production run comprised 200 ps in the case of the pure water simulation and 500 ps in the case of the methane-water system. Configurations were saved every 0.5 ps for analysis...

Figure 2. Comparison of predicted and experimental phase solubilities for methane-water systems (phase liquid—(%) ( 7) vapor—(O) ( 1) (A) (21,22) (-)...

Methane - Water System. Interaction parameters were generated for the vapor phase and the aqueous liquid phase for the methane -... 

A constant interaction parameter was capable of representing the mole fraction of water in the vapor phase within experimental uncertainty over the temperature range from 100°F to 460°F. As with the methane - water system, the temperature - dependent interaction parameter is also a monotonically increasing function of temperature. However, at each specified temperature, the interaction parameter for this system is numerically greater than that for the methane - water system. Although it is possible for this binary to form a three-phase equilibrium locus, no experimental data on this effect have been reported. 

Figure 5. Experimental and predicted vapor and liquid phase compositions for methane-water system at 250°C ((----) P-R prediction (A) (IT) (A) (IS))...

Results of his calculations are listed in table 11.13 in terms of fractionation factors a and AXg. The factors for methane-water vapor and hydrogen-water vapor couples are plotted in figure 11.33 as a function of T (°C). 

Figure 11,33 Calculated oxygen isotope fractionation factors A% for methane-water vapor and hydrogen-water vapor couples plotted against T(°C). Reprinted from Y. Bottinga, Geochimica et Cosmochimica Acta, 33, 49-64, copyright 1969, with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK.

Ohmura, R. Uchida, T. Takeya, S. Nagao, J. Minagawa, H. Ebinuma, T. Narita, H. (2003 a). Clathrate hydrate formation in (methane + water + methylcyclohexanone) systems the first phase equilibrium data. J. Chem. Thermodynamics, 35, 2045-2054. 

Ohmura, R. Matsuda, S. Itoh, S. Ebinuma, T. Narita, H. (2005d). Clathrate Hydrate Crystal Growth in Liquid Water Saturated with a Guest Substance Observations in a Methane + Water System. Crystal Growth Design, 5(3), 953-957. 

The solubility of the components in the solvent must be sufficient. To improve the solubility, cosolvents can be used. Another possibility is the application of a two-phase system or an emulsion in the presence of phase-transfer catalysts. A two-phase system also has advantages in product isolation and continuous electrolysis procedures. A typical example is the synthesis of p-methoxy benzonitrile by anodic substitution of one methoxy group in 1,4-dimethoxybenzene by the cyanide ion (Eq. 22.21). The homogeneous cyanation system (acetonitrile, tetraethylammonium cyanide) [24] can be efficiently replaced by a phase-transfer system (dichloro-methane, water, sodium cyanide, tetrabutylammonium hydrogen sulfate) [71]. 

An activation barrier is associated with the cluster transformation. If the dissolved gas is methane, the barrier for transforming the cluster coordination number from 20 (for the 512) to 24 (for the 51262) is high, both because the guest cannot lend much stability to the larger cavity (see Section 2.1.3.2) and because the 51262 cavities outnumber the 512 in si by a factor of 3. Transformation of methane-water clusters from... 

However, for a multicomponent natural gas mixture, at 5-20 MPa, the subcooling was found to significantly underestimate the driving force (the pure methane-water system showed a far better match between driving force and subcooling). However, above 20 MPa, the driving force was matched well by... 

Figure 3.19 Variations in driving force and subcooling with pressure calculated at constant temperature, T = 273.2 K, for a methane-water hydrate system. (Reproduced from Arjamandi, M., Tohidi, B., Danesh, A., Todd, A.C., Chem. Eng. Sci., 60, 1313 (2005b). With permission from Elsevier.)...

See Section 4.1.5 for other examples of how Gibbs Phase Rule works in the methane + water phase diagram. Section 5.2 shows the application of the Gibbs Phase Rule for hydrate guests of methane, ethane, propane, and their mixtures. 

FIGURE 4.2 Pressure-temperature diagrams, (a) Methane + water or nitrogen + water system in the hydrate region, (b) Hydrocarbon + water systems with upper quadruple points, (c) Multicomponent natural gas + water systems, (d) Hydrocarbon + water systems with upper quadruple points and inhibitors. 

Figure 5.12 Pressure vs. temperature diagram for methane + water system.

Figure 5.12 is the pressure versus temperature phase diagram for the methane+ water system. Note that excess water is present so that, as hydrates form, all gas is incorporated into the hydrate phase. The phase equilibria of methane hydrates is well predicted as can be seen by a comparison of the prediction and data in Figure 5.12 note that the predicted hydrate formation pressure for methane hydrates at 277.6 K is 40.6 bar. 

Figure 5.15 is the pseudo-binary pressure versus excess water composition diagram for the methane+propane+water system at a temperature of 277.6 K. At 277.6 K the hydrate formation pressures are 4.3 and 40.6 bar for pure propane (sll) and pure methane (si) hydrates, respectively, as shown at the excess water composition extremes in Figure 5.15. As methane is added to pure propane, there will be a composition at which the incipient hydrate structure changes from sll to si as seen in the inset of Figure 5.15, this composition is predicted to be 0.9995 mole fraction methane in the vapor—a very small amount of propane added to a methane+water mixture will form sll hydrates.

The most productive two-phase (H-V or H-Lhc) equilibrium apparatus was developed by Kobayashi and coworkers. The same apparatus has been used for two-phase systems such as methane + water (Sloan et al., 1976 Aoyagi and Kobayashi, 1978), methane + propane + water (Song and Kobayashi, 1982), and carbon dioxide + water (Song and Kobayashi, 1987). The basic apparatus described in Section 6.1.1.2 was used in a unique way for two-phase studies. With two-phase measurements, excess gas was used to convert all of the water to hydrate at a three-phase (Lw-H-V) line before the conditions were changed to temperature and pressures in the two-phase region. This requires very careful conditioning of the hydrate phase to prevent metastability and occlusion. Kobayashi and coworkers equilibrated the hydrate phase by using the ball-mill apparatus to convert any excess water to hydrate. 

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